1. Field of the Invention
The present invention relates to systems and methods for transmitting and receiving signals, and in particular to a system and method for use in reducing intercarrier interference in multicarrier communication systems.
2. Description of the Related Art
In recent years, substantial interest has been shown in multi-carrier communication systems. One such multicarrier communication is orthogonal frequency division multiplexing (OFDM), which has been accepted as the new Institute of Electrical and Electronic Engineers (IEEE) wireless local area network standards (IEEE 802.11). OFDM has also been approved for the metropolitan area networks using fixed broadband wireless according to IEEE 802.16.
OFDM offers a very robust transmission method that is somewhat resistant to wireless channel impairments such as multi-path propagation and frequency-selective fading. OFDM also offers increased immunity to impulse noise and fast fading. As an added bonus, OFDM requires less complex equalization, and therefore simplifies receiver design.
Typically, OFDM uses a rectangular subcarrier pulse. This allows the task of pulse forming and modulation to be performed by a simple Inverse Discrete Fourier Transform (IDFT), which can be implemented as an Inverse Fast Fourier Transform (IFFT).
Advantageously, the receiver needs only an FFT to reverse this operation. The time domain's rectangular pulse transforms into a
      sinc    ⁡          (      x      )        =            sin      ⁢                          ⁢      π      ⁢                          ⁢      x              π      ⁢                          ⁢      x      spectrum in the frequency domain.
OFDM takes advantage of the fact that if two interfering signals are placed at a distance of an integer multiple of the symbol frequency, the peak power corresponding to the sinusoidal component of one signal lines up only with zero power components of the other signal. Orthogonal frequency division extends this concept to include a number of carriers, each spaced at the symbol frequency, thus providing maximum spectral efficiency with (ideally) no interference, producing the orthogonal frequency division. FIG. 1 is a diagram showing a frequency domain representation of an OFDM signal orthogonally multiplexing three (102, 104, and 106) signals. These results can be extended to N suitably spaced subcarriers for a data stream with N symbols.
The overlapping subcarrier spectra allows OFDM systems to provide high spectral efficiency. However, the performance of such multicarrier systems is sensitive to synchronization error, such as frequency, time or phase offsets. Such frequency, time or phase offsets can result from carrier frequency synchronization error or from a Doppler shift due to motion between the transmitter and receiver, and can cause a loss of the carriers' orthogonality, and hence create intercarrier interference (ICI).
Four different approaches for mitigating ICI have been proposed. The first approach is that of ICI self cancellation, which is described in Y. Zhao and S-G. Haggman, “Intercarrier interference self-cancellation scheme for OFDM mobile communication systems,” IEEE Trans. Commun., vol. 49, no. 7, pp. 1185-1191, July 2001 (hereinafter referred to as Reference (1)); Y. Zhao and S.-G. Haggman, “Sensitivity to Doppler shift and carrier frequency errors in OFDM systems—The consequences and solutions,” Proc. IEEE 46th Vehicular Technology Conf, Atlanta, Ga., April 1996, pp. 1564-1568 (hereinafter referred to as Reference (2)); and J. Armstrong, “Analysis of new and existing methods of reducing intercarrier interference due to carrier frequency offset in OFDM,” IEEE Trans. Commun., vol. 47, no. 3, March 1999, pp. 365-369 (hereinafter referred to as Reference (3)), all of which are hereby incorporated by reference herein.
The second approach is that of frequency-domain equalization, as described in J. Ahn and H. S. Lee, “Frequency domain equalization of OFDM signal over frequency nonselective Rayleigh fading channels,” Electron. Lett., vol. 29, no. 16, pp. 1476-1477, August 1993 (hereinafter referred to as Reference (5)); and N. A. Dhahi et al., “Optimum finite-length equalization for multicarrier transceivers,” IEEE Trans. Commun., vol. 44, no. 1, pp. 56-64, January 1996 (hereinafter referred to as Reference (6)), both of which are hereby incorporated by reference herein.
The second approach is that of time-domain windowing, as described in R. Li and G. Stette, “Time-limited orthogonal multicarrier modulation schemes,” IEEE Trans. Commun., vol. 43, no. 2/3/4, pp. 1269-1272, February/March/April 1995 (hereinafter referred to as Reference (7)); and C. Muschallik, “Improving an OFDM reception using an adaptive Nyquist windowing,” IEEE Trans. Consumer Electron., vol. 42, pp. 259-269, August 1996 hereinafter referred to as Reference (8)), both of which are hereby incorporated by reference herein.
A third approach involves two-path parallel cancellation schemes, as described in H. G. Yeh and C. C. Wang, “New parallel algorithm for mitigating the frequency offset of OFDM systems,” Proc. IEEE Vehicular Technology Fall Conf., L. A., C A, September 2004 (hereinafter referred to as Reference (9)); and H. G. Yeh and Y. K. Chang, “A conjugate operation for mitigating intercarrier interference of OFDM systems,” Proc. IEEE Vehicular Technology Fall Conf., L. A., C A, September 2004 (hereinafter referred to as Reference (10)), both of which are also hereby incorporated by reference herein.
Finally, frequency offset estimation techniques using training sequence such as pilot symbols are proposed, as described in J.-J van de Beek, M. Sandell, and P. O. Borjesson, “ML estimation of time and frequency offset in OFDM systems,” IEEE Trans. Signal Processing, vol. 45, no. 7, pp. 1800-1805, July. 1997 (hereinafter referred to as Reference (11)), and T. M. Schmidl and D. C. Cox, “Robust frequency and timing synchronization for OFDM,” IEEE Trans Commun., vol. 45, pp. 1613-1621, December 1997 (hereinafter referred to as Reference (12)), both of which are also hereby incorporated by reference herein
However, the foregoing techniques do not readily account for frequency offset estimation errors due to unexpected Doppler shifts due to the relative velocity between the transmitter and receiver (common in moving communication systems), nor any frequency offset errors that are less than five percent of the subcarrier frequency spacing 108, as is typically the case when phase, frequency, and timing synchronization has been accomplished by use of repeated preamble sequences.
There is therefore a need for a method for reducing ICI in multicarrier systems that results from sources that are difficult to predict or model, and which cannot be easily ameliorated with synchronization techniques. The present invention satisfies that need.